The present invention relates to a semiconductor light emitting device used as a light source of an image forming apparatus or an optical printer and a manufacturing method thereof. It more particularly relates to a semiconductor light emitting device which comprises a light emitting diode array and a manufacturing method thereof.
Recently, a light emitting diode array used for a light source of an optical printer or an image forming apparatus using an electrographic process has been studied. This light emitting diode array, which comprises self light-emission type array elements, emits light in accordance with an image signal. The light is applied to a photoconductor via an equal multiplication image forming element to form an electrostatic latent image. The latent image is processed by development, transfer, and fixing, and eventually a printing process is completed by an electrographic method.
A light emitting board 101, included in an optical printer using a conventional light emitting diode array, comprises, as shown in FIG. 1, a base board 102 that serves as a heat sink, and circuit members 103, 104, 105 adhered to the base board 102. Cables 106, 107 are connected to the circuit members 103, 104, 105 in order to supply an electric power source and to supply an image signal. Reference numerals 108.sub.-1 .about.108.sub.-n indicate aligned light emitting diode array chips, and reference numerals 109.sub.-1 .about.109.sub.-n and 110.sub.-1 .about.110.sub.-n indicate driver circuits that drive the light emitting diode array chips 108.sub.-1 .about.108.sub.-n. Each of the chips is a driving IC for light emitting diodes incorporated with a serial/parallel converter for an image signal supplied from the cable 106, 107.
In the above mentioned light emitting board 101, image signals for a single line are appropriately supplied to the driver circuits 109.sub.-1 .about.109.sub.-n and 110.sub.-1 .about.110.sub.-n. After the data for the entire portion of the single line is supplied to the driver circuits, the data is supplied to the drive terminals of the light emitting diodes. According to the data, each light emitting diode turns o and off and image forming lighting points are generated for one single line.
As shown in FIG. 2, a light emitted from a light emitting diode 201.sub.-m-p of each light emitting diode 201.sub.-m is projected onto a photoconducting drum 203 via an equal multiplication image forming system 202 such as a SELFOC lens array or a RMLA (roof mirror lens array).
The size of an optical printer head using such a light emitting diode array can be minimized because there are no moving parts and it has a reduced number of component parts Additionally, since the light emitting diode is a self lighting type and has a high quenching ratio, a high contrast image is obtained. Further advantages can also be obtained, such as a possibility of making a longer array by connecting a number of chips, and a possibility of making a high speed array by increasing an output of the light emitting diodes.
There are two different type of light emitting diodes used for optical printer heads. One is a plane surface lighting type light emitting diode array which comprises a number of light emitting portions, having a square face for example, arranged in a plane parallel to a base board. The other is an edge surface lighting type light emitting diode array which emits a light from an edge surface perpendicular to a plane surface of a base board.
An example of the basic construction of a plane surface lighting type light emitting diode array, shown in FIG. 3, is suggested in the preliminary draft of the 1980 session of the institute of Electronics and Communication Engineers of Japan. In this plane surface lighting type light emitting diode array, electrodes 121 are formed on both sides of, or around the light emitting portion 120 so that intensity of the light from the light emitting portion 120 becomes uniform in a light emitting plane. However, in this construction, a width of each element becomes a sum of a width of the light emitting portion 120, a width of the electrode 121, and a width of the separation area between the elements. Thus, forming of a high density light emitting portion, for example more than 600 dpi (dots per inch), is extremely difficult.
An example of the basic construction of an edge surface lighting type light emitting diode array, shown in FIG. 4, is suggested in Japanese Laid-Open Patent Application No. 60-32373. In this example, a plurality of light emitting portions 122 are formed within a layered construction on a base board. These light emitting portions 122 are electrically and physically separated by separation grooves formed to be perpendicular to a surface of the base board. As shown in FIG. 4, the light emitting portions 122 and the electrodes 124, 125 are not placed in the same plane, thus a with of each element is a sum of a width of a light emitting portion 122 and a width of the separation area between the elements. Thus, forming of a high density light emitting portion, for example more than 600 dpi (dots per inch), is possible.
Therefore, it is said that an edge surface lighting type light emitting diode array is suitable for a light emitting diode array used as a high density light source for optical printers. With respect to a dispersion of light emitted from each light emitting diode in a light emitting diode array, a good uniformity of a thickness of a crystal layer and uniformity of an electrical and an optical characteristics of a film in a base board have been achieved by the recent progress of technology in crystal growth of compound semiconductors. Accordingly, it has become possible to have a dispersion of light emitted from each light emitting diode of a light emitting diode array on the same chip falls within .+-.5%.
In a light emitting device using the above mentioned light emitting diode array, differences in intensity of light are produced between chips due to the non-uniformity of mounting which results in a non-uniformity of heat radiation, and electrode forming means for producing a characteristic of crystal layer of a compound semiconductor which comprises the light emitting diode array. This results in large differences in contrast or size of dots when used as a light source of an optical printer.
Particularly, in the conventional edge surface lighting type light emitting diode array, as shown in FIG. 4, a light emitting edge surface is formed by cleaving. Producing the light emitting edge surface by cleaving results in various shapes of the light emitting edge surface because the cleaving itself is not a well established technology. Accordingly, differences are produced of scattering and absorbing of the light at the light emitting surface. These differences result in lack of uniformity in the intensity of lights emitted from the light emitting diode array. Since the light emitting edge surface is formed by cleaving, the light emitting edge surface inevitably becomes an edge of the base board. Accordingly, the light emitting surface is easily damaged while handling the light emitting diode array for mounting; thus there are problems in that uniformity of intensity of the light is further reduced and in that the yield of production is low.
As a rudimentary way of eliminating the above mentioned disadvantages, as shown in FIG. 5, it has been considered to form a light emitting edge surface by a method other than cleaving, and to form a terrace 29 under the light emitting edge surface. In this way, the light emitting edge surface can be formed with a uniform shape because the light emitting edge surface is formed by the same separation grooves between elements are formed; the terrace 29, which allows the light emitting edge surface to not become an edge of the base board, is also formed together with the grooves. Therefore, uniformity in the intensity of lights is obtained and yield of production can be increased.
In case a light emitting diode array is used as a light source of an optical printer, as shown in FIG. 2, a light emitted from a light emitting diode 201.sub.-m-p is focused on a surface of a photoconducting drum 203 via an equal multiplication image forming system 202. In such an optical system, an angle .THETA. from a center axis of a light beam having passed through a lens, to an edge of said light beam (=arctan(.phi./2s):.phi. is an effective diameter of the lens, S is a distance of an object) is important. That is, a number of openings of an equal multiplication image forming element (N.A.=n.multidot.sin .THETA.:n is an index of reflection of the medium) is determined by considering MTF (a spatial frequency of the lens). In case an diverging angle .theta. from the center axis of a light beam emanating from a light emitting diode to an edge of the light beam is larger than .THETA., an efficiency of utilizing light is decreased due to the beam not being able to enter into the equal multiplication image forming element. Therefore, an effort has been made to decrease the angle .theta..
Generally, it is known that the diverging angle .theta., from a center axis of the beam emitted from an edge surface lighting type light emitting diode to an edge of the light beam is smaller than that of the plane surface lighting type because, in an edge surface lighting type light emitting diode, light emission from near an edge surface mainly contributes to the intensity of light in contrast to emission from an inner side.
According to the results of experimental work by the present applicant, an angle 2.theta., from edge to edge of a width of a light beam of an edge surface lighting type light emitting diode was measured as 30.degree..about.100.degree., while that of a plane surface lighting type was measured as approximately 120.degree.. It is assured that making an element having an angle of light beam within this angle range is possible.
When simultaneously forming a light emitting edge surface and separation grooves between adjacent elements on a light emitting diode, a shape of a base board in front of a light emitting edge surface is to be designed so as to satisfy the following equation (1): EQU Lx&lt;Lz/tan .theta. (1)
where the parameters of the above equation (1), shown in FIG. 5, are:
Lz: a depth from a light emitting layer 23 to a terrace surface 29a
Lx: a length between a light emitting edge surface and a terrace edge 1a
.theta.: a diverging angle of light beam emitted from a light emitting edge surface.
As far as the elevation angle .theta.is concerned, no problem may arise if the equation (1) is not satisfied. Concerning the depression angle.theta., as shown in FIG. 7A, if a length L x between a light emitting edge surface and an edge of the base board (an edge of a terrace 1) is too long, a light beam emitted from the light emitting edge surface is reflected by the top surface of the terrace 29 and the reflected light beam proceeds in a direction of the elevation angle.theta.. As a result, the angle 2.theta. increases because of interference of the light beam emitted in the direction of the elevation angle.theta. with the light beam reflected by the terrace surface 29a; thus the angle 2 .theta. becomes larger than the angle 2.THETA. of an equal multiplication image forming element, thus reducing an efficiency of utilization of light.
When L z is too short, as shown in FIG. 7B, or when the angle.theta. is too large, as shown in FIG. 7C, the same phenomena occurs and efficiency of utilization of light is decreased. Accordingly, it is very important to design the base board in front of the light emitting edge surface of an edge surface lighting type light emitting diode array so as to satisfy the equation (1) so that the angle .theta. is effectively controlled with respect to the angle.THETA. of the equal multiplication image forming element.
When the base board in front of the light emitting edge surface of an edge surface lighting type light emitting diode array is made based on a design satisfying the equation (1), there still remain the following problems in production. In a conventional production process of a compound semiconductor element, usually a 2 inch single crystal wafer is used, and light emitting devices such as the above mentioned edge surface lighting type light emitting diode array are produced by efficiently arranging a plurality of arrays on the wafer, as shown in FIG. 6. Each of the light emitting diode arrays are then cut away along an inscribed line 30 by cleaving technology or dicing technology.
Therefore, accuracy in cutting the wafer is an important factor for satisfying the equation (1). However, as cleaving is not yet an established technology, it is difficult to obtain good accuracy for forming the L x . Although dicing technology is a more stable technology than cleaving technology, the wafer tends to chip due to the high rotation speed of a disc blade, as high as 30,000 rpm, used to cut the wafer; thus it is difficult to obtain good accuracy without defects in the cut away chip.
Table 1 shows the results of calculations in accordance with the equation Lx=Lz/tan .theta.; .theta.=15.degree. and 50.degree..
TABLE 1 ______________________________________ .theta. 15.degree. 50.degree. ______________________________________ Lz 1.0 2.7 5.0 10.0 1.0 5.0 10.0 12.0 Lx 3.7 10.0 18.6 37.3 0.8 4.1 8.3 10.0 (.mu.m) ______________________________________
By means of the table 1, it is understood, for example, the case the angle .theta. is 15.degree. and the depth Lz is 1 .mu.m; the reflection at a terrace can be prevented by having an Lx of 3.7 .mu.m. Similarly, when the angle .theta. is 50.degree., Lx is to be 0.8 .mu.m with an Lz of 1 .mu.m. Accordingly, high accuracy is required when cutting the wafer with respect to the size of Lx.
According to an experiment by the present applicant, Lx can be formed as short as approximately 10 .mu.m, by using dicing technology. When Lx is 10 .mu.m and the angle .theta. is 50.degree., Lz needs to be more than 12 .mu.m. However, etching of 12 .mu.m requires a long process time ( approx. 10 min./.mu.m ) that results in an increase in the manufacturing costs. Additionally, there is a problem in that if a step between a light emitting element and a metal lead pattern portion is more than 12 .mu.m, the metal lead tends to be easily cut. Therefore, it is preferable to limit techniques to the forming of Lz , but rather to investigate other methods of solving this problem.
On the other hand, in case Lx=10 .mu.m and the angle .theta. is 15.degree., the dry etching process does not take a long time because Lz is required to be only 2.7 .mu.m. The angle .theta. is controlled in accordance with epitaxial growth technology, such as the MOCVD method, for forming of layers of an element structure. However, to achieve a condition where the angle .theta. is less than 15.degree., all of the edge surface lighting type light emitting diode arrays on a wafer (on the entirety of the 2 inch wafer) are required to be formed so as to satisfy optically transmissible conditions and a thickness of an active layer has to be uniformly less than 500 .ANG.. The technique for realizing the angle .theta. of less than 15.degree. has been achieved experimentally, however, it is difficult to introduce the technique into mass production.